1. Field of the Invention
The present invention relates to a magneto-optic disc drive apparatus and, more particularly, to a recording medium drive apparatus having a function whereby in the case of reproducing a magneto-optic disc on which pit information and magnetization information are recorded in a mixed fashion, the reproduced information can be accurately processed irrespective of the polarities of the pit information and magnetization information.
2. Description of the Prior Art
In recent years, the amount of information being processed by computers is steadily increasing. Much attention has been focused on magneto-optic disc apparatus which can record, reproduce and erase large amounts of information. The improvements which have been achieved in this technique are remarkable.
With magneto-optic disc apparatus, there is no need for the user to change the address information of tracks and/or sectors on the recording medium, sector marks allowing the user to recognize the heads of the sectors, and the like. They are therefore previously recorded as concave or convex pit information when the discs are manufactured. On the other hand, data such as user's data in the area which needs to be rewritten is processed as magnetization information.
The amplitudes and polarities of the pit information reproduction signals and magnetization information reproduction signals will now be simply explained. The amplitude of a pit information reproduction signal depends on the light power on the medium surface, light reflectance of the medium, modulation degree according to the depths of convex or concave pits, and/or light transmittance of the reproduction optical system. The amplitude of the magnetic information reproduction signal depends on the light power on the medium surface, light reflectance of the medium, Kerr rotational angle, and/or light transmittance of the reproduction optical system. In addition, the polarity of the magnetization information reproduction signal depends on the magnetizing direction of the magnetized domain.
An example of a conventional apparatus for reproducing a disk on which such different kinds of mixed signals are recorded is disclosed in Japanese Patent Application No. 61-267953.
FIG. 1 is a diagram showing the arrangement of a conventional magneto-optic disc apparatus. In the diagram, a magneto-optic disc 1 is rotated by a rotating motor 2. A perpendicular magnetization film having a magneto optical effect is formed on the disc 1. The recording, erasure and reproduction of the magnetization information are performed in the following manner.
The light emitted from a semiconductor laser 3 is converted into the parallel light fluxes by a coupling lens 4 and transmitted to a condenser lens 6 through a polarizing prism 5. Then, the light fluxes are focused as a micro spot onto the perpendicular magnetization film on the disc 1. The light reflected from the disc 1, whose polarizing plane is rotated, passed through the condenser lens 6 and polarizing prism 5 and is led to an analyzer 8. The analyzer 8 is an optical device adapted to allow only a special polarizing component to pass through it. Therefore, the analyzer 8 can convert the rotation of the polarizing plane into a change in amount of light. This light amount is converted into an electric signal by a photodetector 9 and, thereafter, it is amplified to a desired level by an amplifier 40. The principle of the information reproduction that is effected with such a detection optical system will be described by reference to FIG. 2, mainly with respect to the detection of the rotation of the polarizing plane of the analyzer.
In FIG. 2, an axis 21 is a polarizing axis of a laser beam which is irradiated onto the disc 1. Assuming that the polarizing plane of the light reflected by the disc 1 was only rotated by, e.g., a Kerr rotational angle of .theta..sub.k in the portion having the magnetized domains, the polarizing plane in the non-recorded portion is rotated by a Kerr rotational angle of -.theta..sub.k. An axis 22 which is rotated by an angel of 90.degree. from the axis 21 is called a quenching axis. When the polarizing plane passing axis of the analyzer 8 is made coincident with the quenching axis 22, the amount of light which passes through the analyzer 8 in the polarizing state of the axis 21 reaches its minimum value.
When the polarizing plane passing axis of the analyzer 8 is only rotated by an angle of .theta..sub.A from the quenching axis 22 and set, the amount of light which passes through the analyzer is equal to the amount of light projected on the .theta..sub.A axis. Namely, the change in light amount corresponding to the presence or absence of the magnetized domain is derived as a magnetization information reproduction signal.
When the rotational angle of the analyzer is set such that the S/N (signal to noise) ratio of the magnetization information reproduction signal reaches its maximum value, the amplitudes of tho pit information reproduction signal generally differ from those of the magnetization information reproduction signal by several fold.
Therefore, if the magnetization information reproduction signal and pit information reproduction signal are binarized by using different threshold values, only the pit information reproduction signal can be read out. Further, if the OR of the logic data derived by binarizing those signals by use of the different threshold values is calculated, they can also be reproduced as a series of data trains.
FIG. 3 is a time chart showing binarizing and synthesizing methods. The threshold values for binarization in comparators 41 and 42 are set as shown in the diagram for the signal from the amplifier 40. Thus, only the pit information reproduction signal is output as the binarization signal from the comparator 41. In the binarization signal from the comparator 42, the pit information reproduction signal and magnetization information reproduction signal exist in a mixed state. To output a series of data trains, the OR of the outputs of the comparators 41 and 42 is calculated.
On the other hand, by setting the rotating direction of the analyzer to the direction opposite to that in FIG. 3, a signal as shown in FIG. 4 is obtained as the output of the amplifier 40. Therefore, by respectively setting the threshold values of the comparators 41 and 42 as shown in FIG. 4, each binarization signal corresponds solely to the pit information reproduction signal or the magnetization information reproduction signal. If those signals are processed as a series of data trains, the OR of the outputs of the comparators 41 and 42 is calculated by an OR circuit 43 in a manner similar to the case of FIG. 3. According to the above constitution, when reproducing the information from a magneto-optic recording medium on which the pit information reproduction signal and magnetization information reproduction signal are recorded in a mixed manner, the polarizing plane passing axis angle of the analyzer is set so as to obtain the maximum S/N ratio of the magnetization signal. For the signal obtained by the photodetector arranged after the analyzer, different threshold values are respectively set for the pit information reproduction signal and magnetization information reproduction signal, and binarization is performed by using these threshold values. This offers such advantages that the detection optical system can be miniaturized and simplified and the information can be reproduced without any data dropout.
The conventional recording medium drive apparatus is constituted in the manner explained above.
In general, the recording medium drive apparatus permits fluctuation components of various kinds of parameters such as reflectance of the recording medium, laser power, Kerr rotational angle, light transmittance of the reproduction optical system, and the like. It is necessary to accurately reproduce the information. Among the various kinds of parameters, those whose fluctuation ranges are already known will now be mentioned. The product (R.times..theta..sub.x) of a reflectance R of the recording medium and the Kerr rotational angle .theta..sub.k fluctuates within a range of from 0.1 to 0.25. The laser power fluctuates within a range of from 0.4 to 15 mW in consideration of the recording, reproduction and erasure. Thus, it can be predicted that the output of a photodetector fluctuates to a remarkable extent.
On the other hand, a recording medium drive apparatus must also cope with changes in polarities of pit information reproduction signals and magnetization information reproduction signals.
However, in the conventional apparatus, since the gain of the amplifier 40 and the threshold values of the comparators 41 and 42 are fixed, it is difficult to accurately reproduce the information for any large fluctuation in input signal which is caused by fluctuations in the parameters mentioned above. Further, the polarity of the pit information reproduction signal cannot be switched over. Therefore, there are limitations in terms of the recording medium and laser power which can be used and the versatility of a conventional apparatus is therefore lacking.